By having high energy density, high durability and charging and discharging efficiency, a non-aqueous electrolyte secondary battery, in particular a lithium ion secondary battery, has been widely used as a power source for a mobile device.
Against the background of escalating environmental protection movement in recent years, the application range of a lithium ion secondary battery is broadened for a power source of a large-size system like an electromotive vehicle or a stationary power storage system. With regard to the battery used as such power sources, it is required to have large battery size, improved battery characteristics like high energy density, and high production efficiency for having those battery performances in an economically favorable way.
In a lithium ion secondary battery, when charging and discharging of battery is repeated, a non-aqueous solvent in an electrolyte is decomposed by reacting electrochemically with a negative electrode active material and decomposition products generated by the reaction are deposited as a SEI (Solid Electrolyte Interface) film on a surface of a negative electrode active material. As a result, a problem arises that increased reaction resistance of an electrode is yielded. Furthermore, since the SEI film thus deposited is generally a hard inorganic film, there is also a problem that it may be broken by stress caused by expansion and shrinking of a negative electrode active material during charging and discharging, and thus the characteristics of an electrode are deteriorated.
For such reasons, it is known to add an additive to an electrolyte of a lithium ion secondary battery, and by intentional decomposition of an additive on an electrode surface during initial charging, the decomposition product can function as a protective SEI film for suppressing decomposition of new electrolyte. The SEI film based on the additive is known to exhibit a significant influence on cycle performance, storage performance, charging and discharging efficiency, and safety of a battery.
It is described in JP 2001-325988 A (US 2002/034,678 A) that, plural kinds of non-aqueous solvent are included and an initial charging is performed by a charging step having a step in which charging is performed at a potential at which one non-aqueous solvent is reduced and a step in which charging is performed at a potential at which another non-aqueous solvent is reduced, a favorable SEI film can be formed.